A vast quantity of raw European census microdata for the period since the 1960s survives in machine-readable form. Most of these data, however, remain inaccessible to researchers. This proposal seeks funding to create harmonized and documented samples of over 50 Western and Eastern European censuses. These data will be made available for scholarly and educational research through a web-based data dissemination system. This project leverages previous federal investments in social science infrastructure. Grants from the National Institutes of Health and the National Science Foundation have laid the groundwork for the European data series by funding many of the initial costs. Those projects have underwritten the development of data cleaning and sampling procedures, data conversion and dissemination software, and design protocols for data and documentation. We have already made arrangements to obtain raw microdata files, internal documentation, and redistribution agreements for over 60 censuses of 17 European countries with populations totaling one-half billion people. As a result, the new public-use microdata samples for Europe will be highly cost-effective. The following tasks must be carried out to capitalize on these past investments and make the European data widely available to researchers: draw new samples of each census; reformat and clean the samples; impose confidentiality protections; recode variables into existing harmonized coding systems and develop new coding designs optimized for Europe; allocate missing and inconsistent data values; create a set of consistent constructed variables; develop harmonized English-language documentation; convert all documentation to the Data Documentation Initiative metadata standard; and improve and maintain the web-based data access system. With over 70 million records spanning a forty-year period, the new database will allow social scientists to make comparisons across European nations during decades of dramatic change. Coupled with data from other IPUMS projects, this information will allow innovative comparative research across time and space. The data series will result in a substantial body of new scientific and policy-relevant research on economic transformation, demographic transition and population aging, international migration, and many other topics.